Storage system with decrement protection of reference counts

ABSTRACT

A storage system in one embodiment comprises a plurality of storage devices storing data pages. Each data page has a content-based signature derived from that data page. The content-based signatures are associated with physical locations storing the data pages. In response to receipt of a write input/output (IO) request that includes a data segment that is smaller than a page granularity of the storage devices, a content-based signature associated with the data segment is determined which also corresponds to a target data page stored at one of the physical locations. In response to determining the content-based signature, an inflight write count corresponding to the content-based signature is incremented. In response to a decrement request to decrement a reference count of the physical location corresponding to the content-based signature, a decrement flag corresponding to the content-based signature is set in the data structure and the decrement request is postponed.

FIELD

The field relates generally to information processing systems, and moreparticularly to storage in information processing systems.

BACKGROUND

In some information processing systems, volatile write cache temporarilystores or caches data to be later written to a persistent data storagelocation (i.e., destaged) during a background destaging process. Theinformation processing system may often have a fixed-size pagegranularity and the system may support write input/output (IO) requestsfor data segments smaller than the system's page granularity, i.e.,small write requests. When a small write request is received, the writecache temporarily stores the new segment data associated with the smallwrite request for later destaging in a write cache destaging process.During the destaging process, the new segment data associated with thesmall write request is hardened. For example, the data in the data pagetargeted by the small write request may be read and combined with thenew segment data of the small write request to form a new data pagewhich then is stored in the persistent data storage location. A receivedwrite request is considered a pending or “inflight” write request priorto being stored in the persistent data storage location, e.g., whileawaiting or being processed in the destaging process.

In some systems, the data pages in the persistent data storage locationmay each have an associated reference count that indicates the number ofreferences to that page in an address-to-hash (A2H) mapping of theinformation processing system. The reference count for a given data pagemay be updated as the number of references to that given data pageincreases or decreases. For example, increment (“Incref”) and decrement(“Decref”) commands may be issued to increment or decrement thereference count associated with a data page in the persistent datastorage location.

When the reference count for a given data page is decremented to zero,the given data page may be removed or marked for removal since the datapage is no longer used by the system.

SUMMARY

Illustrative embodiments provide techniques for decrement protection ofreference counts for inflight small write requests in a storage system.

In one embodiment, a storage system comprises a plurality of storagedevices and an associated storage controller. The plurality of storagedevices are configured to store a plurality of data pages. Each of thedata pages has a content-based signature derived from content of thatdata page. The content-based signatures of the data pages are associatedwith physical locations in the plurality of storage devices where thedata pages are stored. The plurality of storage devices store areference count for each physical location. A given reference countindicates a number of the data pages that map via their respectivecontent-based signatures to the same physical location in the pluralityof storage devices.

The storage controller is configured to receive a write input/output(IO) request. The write IO request includes a data segment that issmaller than a page granularity of the plurality of storage devices.

In response to receiving the write IO request, the storage controller isconfigured to determine a content-based signature associated with thedata segment. The content-based signature corresponds to a target datapage stored at one of the physical locations.

In response to a decrement request to decrement a reference count of thephysical location corresponding to the content-based signature of thetarget data page, the storage controller is configured to postpone thedecrement request.

The storage controller may be implemented using at least one processingdevice comprising a processor coupled to a memory.

In some embodiments, the storage controller may be further configured toincrement an inflight write count corresponding to the determinedcontent-based signature of the target data page in a data structureassociated with the storage controller in response to determining thecontent-based signature associated with the data segment.

The storage controller may be further configured to decrement theinflight write count in response to completion of the received write IOrequest. The storage controller may be further configured to execute thepostponed decrement request in response to the inflight write countbeing decremented to a predetermined value.

In some embodiments, in response to the decrement request, the storagecontroller may be further configured to set a decrement postponed flagcorresponding to the content-based signature of the target data page ina data structure associated with the storage controller.

In response to a second decrement request to decrement the referencecount of the physical location corresponding to the content-basedsignature of the target data page, the storage controller may be furtherconfigured to determine whether the decrement postponed flagcorresponding to the content-based signature of the target data page isset in the data structure. In response to determining that the decrementpostponed flag corresponding to the content-based signature of thetarget data page is set in the data structure, the storage controllermay be further configured to decrement the reference count of thephysical location corresponding to the content-based signature of thetarget data page.

In some embodiments, in response to a recovery of the storage systemafter an event, the storage controller may be further configured toreset the data structure. The storage controller may be furtherconfigured to determine whether any recovered write IO requests includea data segment smaller than a page granularity of the plurality ofstorage devices and, for a given write IO request that includes a datasegment smaller than a page granularity of the plurality of storagedevices, the storage controller maybe further configured to incrementthe inflight write count corresponding to the content-based signature ofa data page targeted by the given write IO request in the datastructure. The storage controller may be further configured to determinewhether the decrement request postponed journal includes a decrementrequest corresponding to the content-based signature of the data pagetargeted by the given write IO request. In response to determining thatthe decrement request postponed journal includes a decrement requestcorresponding to the content-based signature of the data page targetedby the given write IO request, the storage controller may be furtherconfigured to set the decrement postponed flag corresponding to thecontent-based signature of the data page targeted by the given write IOrequest in the data structure.

These and other illustrative embodiments include, without limitation,apparatus, systems, methods and processor-readable storage media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an information processing system comprisinga content addressable storage system configured with functionality fordecrement protection of reference counts for inflight small writerequests in an illustrative embodiment.

FIG. 2 shows an example of a set of user data pages in an illustrativeembodiment.

FIG. 3 shows an example of a set of metadata pages in an illustrativeembodiment.

FIG. 4 shows an example of a decref hash table in an illustrativeembodiment.

FIGS. 5A-5C are flow diagrams of portions of a process for decrementprotection of reference counts for inflight small write requests in anillustrative embodiment.

FIGS. 6 and 7 show examples of processing platforms that may be utilizedto implement at least a portion of an information processing system inillustrative embodiments.

DETAILED DESCRIPTION

Illustrative embodiments will be described herein with reference toexemplary information processing systems and associated computers,servers, storage devices and other processing devices. It is to beappreciated, however, that these and other embodiments are notrestricted to the particular illustrative system and deviceconfigurations shown. Accordingly, the term “information processingsystem” as used herein is intended to be broadly construed, so as toencompass, for example, processing systems comprising cloud computingand storage systems, as well as other types of processing systemscomprising various combinations of physical and virtual processingresources. An information processing system may therefore comprise, forexample, at least one data center or other cloud-based system thatincludes one or more clouds hosting multiple tenants that share cloudresources. Numerous other types of enterprise computing and storagesystems are also encompassed by the term “information processing system”as that term is broadly used herein.

FIG. 1 shows an information processing system 100 configured inaccordance with an illustrative embodiment. The information processingsystem 100 comprises a computer system 101 that includes compute nodes102-1, 102-2, . . . 102-N. The compute nodes 102 communicate over anetwork 104 with a content addressable storage system 105. The computersystem 101 is assumed to comprise an enterprise computer system or otherarrangement of multiple compute nodes associated with respective users.

The compute nodes 102 illustratively comprise respective processingdevices of one or more processing platforms. For example, the computenodes 102 can comprise respective virtual machines (VMs) each having aprocessor and a memory, although numerous other configurations arepossible.

The compute nodes 102 can additionally or alternatively be part of cloudinfrastructure such as an Amazon Web Services (AWS) system. Otherexamples of cloud-based systems that can be used to provide computenodes 102 and possibly other portions of system 100 include Google CloudPlatform (GCP) and Microsoft Azure.

The compute nodes 102 may be viewed as examples of what are moregenerally referred to herein as “host devices” or simply “hosts.” Suchhost devices are configured to write data to and read data from thecontent addressable storage system 105. The compute nodes 102 and thecontent addressable storage system 105 may be implemented on a commonprocessing platform, or on separate processing platforms. A wide varietyof other types of host devices can be used in other embodiments.

The compute nodes 102 in some embodiments illustratively provide computeservices such as execution of one or more applications on behalf of eachof one or more users associated with respective ones of the computenodes 102.

The term “user” herein is intended to be broadly construed so as toencompass numerous arrangements of human, hardware, software or firmwareentities, as well as combinations of such entities. Compute and/orstorage services may be provided for users under a platform-as-a-service(PaaS) model, although it is to be appreciated that numerous other cloudinfrastructure arrangements could be used. Also, illustrativeembodiments can be implemented outside of the cloud infrastructurecontext, as in the case of a stand-alone enterprise-based computing andstorage system.

Such users of the storage system 105 in some cases are referred toherein as respective “clients” of the storage system 105.

The network 104 is assumed to comprise a portion of a global computernetwork such as the Internet, although other types of networks can bepart of the network 104, including a wide area network (WAN), a localarea network (LAN), a satellite network, a telephone or cable network, acellular network, a wireless network such as a WiFi or WiMAX network, orvarious portions or combinations of these and other types of networks.The network 104 in some embodiments therefore comprises combinations ofmultiple different types of networks each comprising processing devicesconfigured to communicate using Internet Protocol (IP) or othercommunication protocols.

As a more particular example, some embodiments may utilize one or morehigh-speed local networks in which associated processing devicescommunicate with one another utilizing Peripheral Component Interconnectexpress (PCIe) cards of those devices, and networking protocols such asInfiniBand, Gigabit Ethernet or Fibre Channel. Numerous alternativenetworking arrangements are possible in a given embodiment, as will beappreciated by those skilled in the art.

The content addressable storage system 105 is accessible to the computenodes 102 of the computer system 101 over the network 104. The contentaddressable storage system 105 comprises a plurality of storage devices106, an associated storage controller 108, and an associated cache 109.The storage devices 106 are configured to store metadata pages 110 anduser data pages 112, and may also store additional information notexplicitly shown such as checkpoints and write journals. The metadatapages 110 and the user data pages 112 are illustratively stored inrespective designated metadata and user data areas of the storagedevices 106. Accordingly, metadata pages 110 and user data pages 112 maybe viewed as corresponding to respective designated metadata and userdata areas of the storage devices 106.

A given “page” as the term is broadly used herein should not be viewedas being limited to any particular range of fixed sizes. In someembodiments, a page size of 8 kilobytes (KB) is used, but this is by wayof example only and can be varied in other embodiments. For example,page sizes of 4 KB, 16 KB or other values can be used. Accordingly,illustrative embodiments can utilize any of a wide variety ofalternative paging arrangements for organizing the metadata pages 110and the user data pages 112.

The user data pages 112 are part of a plurality of logical units (LUNs)configured to store files, blocks, objects or other arrangements ofdata, each also generally referred to herein as a “data item,” on behalfof users associated with compute nodes 102. Each such LUN may compriseparticular ones of the above-noted pages of the user data area. The userdata stored in the user data pages 112 can include any type of user datathat may be utilized in the system 100. The term “user data” herein istherefore also intended to be broadly construed.

It is assumed in the present embodiment that the storage devices 106comprise solid state drives (SSDs). Such SSDs are implemented usingnon-volatile memory (NVM) devices such as flash memory. Other types ofNVM devices that can be used to implement at least a portion of thestorage devices 106 include non-volatile random access memory (NVRAM),phase-change RAM (PC-RAM) and magnetic RAM (MRAM). Various combinationsof multiple different types of NVM devices may also be used.

However, it is to be appreciated that other types of storage devices canbe used in other embodiments. For example, a given storage system as theterm is broadly used herein can include a combination of different typesof storage devices, as in the case of a multi-tier storage systemcomprising a flash-based fast tier and a disk-based capacity tier. Insuch an embodiment, each of the fast tier and the capacity tier of themulti-tier storage system comprises a plurality of storage devices withdifferent types of storage devices being used in different ones of thestorage tiers. For example, the fast tier may comprise flash driveswhile the capacity tier comprises hard disk drives. The particularstorage devices used in a given storage tier may be varied in otherembodiments, and multiple distinct storage device types may be usedwithin a single storage tier. The term “storage device” as used hereinis intended to be broadly construed, so as to encompass, for example,flash drives, solid state drives, hard disk drives, hybrid drives orother types of storage devices.

In some embodiments, the content addressable storage system 105illustratively comprises a scale-out all-flash storage array such as anXtremIO™ storage array from Dell EMC of Hopkinton, Mass. Other types ofstorage arrays, including by way of example VNX® and Symmetrix VMAX®storage arrays also from Dell EMC, can be used to implement storagesystems in other embodiments.

The term “storage system” as used herein is therefore intended to bebroadly construed, and should not be viewed as being limited to contentaddressable storage systems or flash-based storage systems. A givenstorage system as the term is broadly used herein can comprise, forexample, network-attached storage (NAS), storage area networks (SANs),direct-attached storage (DAS) and distributed DAS, as well ascombinations of these and other storage types, includingsoftware-defined storage.

Other particular types of storage products that can be used inimplementing a given storage system in an illustrative embodimentinclude all-flash and hybrid flash storage arrays such as Unity™,software-defined storage products such as ScaleIO™ and ViPR®, cloudstorage products such as Elastic Cloud Storage (ECS), object-basedstorage products such as Atmos®, and scale-out NAS clusters comprisingIsilon® platform nodes and associated accelerators, all from Dell EMC.Combinations of multiple ones of these and other storage products canalso be used in implementing a given storage system in an illustrativeembodiment.

The content addressable storage system 105 in the embodiment of FIG. 1is configured to generate hash metadata providing a mapping betweencontent-based digests of respective ones of the user data pages 112 andcorresponding physical locations of those pages in the user data area.Content-based digests generated using hash functions are also referredto herein as “hash digests.” Such hash digests or other types ofcontent-based digests are examples of what are more generally referredto herein as “content-based signatures” of the respective user datapages 112. The hash metadata generated by the content addressablestorage system 105 is illustratively stored as metadata pages 110 in themetadata area.

The generation and storage of the hash metadata is assumed to beperformed under the control of the storage controller 108. The hashmetadata may be stored in the metadata area in a plurality of entriescorresponding to respective buckets each comprising multiple cachelines, although other arrangements can be used. In some aspects, thehash metadata may also be loaded into cache 109.

Each of the metadata pages 110 characterizes a plurality of the userdata pages 112. For example, as illustrated in FIG. 2, a given set ofuser data pages 200 representing a portion of the user data pages 112illustratively comprises a plurality of user data pages denoted UserData Page 1, User Data Page 2, . . . User Data Page n. Each of the userdata pages in this example is characterized by a LUN identifier, anoffset and a content-based signature. The content-based signature isgenerated as a hash function of content of the corresponding user datapage. Illustrative hash functions that may be used to generate thecontent-based signature include SHA1, where SHA denotes Secure HashingAlgorithm, or other SHA protocols known to those skilled in the art. Thecontent-based signature is utilized to determine the location of thecorresponding user data page within the user data area of the storagedevices 106 of the content addressable storage system 105.

Each of the metadata pages 110 in the present embodiment is assumed tohave a signature that is not content-based. For example, the metadatapage signatures may be generated using hash functions or other signaturegeneration algorithms that do not utilize content of the metadata pagesas input to the signature generation algorithm. Also, each of themetadata pages is assumed to characterize a different set of the userdata pages.

This is illustrated in FIG. 3, which shows a given set of metadata pages300 representing a portion of the metadata pages 110 in an illustrativeembodiment. The metadata pages in this example include metadata pagesdenoted Metadata Page 1, Metadata Page 2, . . . Metadata Page m, havingrespective signatures denoted Signature 1, Signature 2, . . . Signaturem. Each such metadata page characterizes a different set of n user datapages. For example, the characterizing information in each metadata pagecan include the LUN identifiers, offsets and content-based signaturesfor each of the n user data pages that are characterized by thatmetadata page. It is to be appreciated, however, that the user data andmetadata page configurations shown in FIGS. 2 and 3 are examples only,and numerous alternative user data and metadata page configurations canbe used in other embodiments.

The content addressable storage system 105 in the FIG. 1 embodiment isimplemented as at least a portion of a clustered storage system andincludes a plurality of storage nodes 115 each comprising acorresponding subset of the storage devices 106. Other clustered storagesystem arrangements comprising multiple storage nodes can be used inother embodiments. A given clustered storage system may include not onlystorage nodes 115 but also additional storage nodes 120 coupled tonetwork 104. Alternatively, the additional storage nodes 120 may be partof another clustered storage system of the system 100. Each of thestorage nodes 115 and 120 of the system 100 is assumed to be implementedusing at least one processing device comprising a processor coupled to amemory.

The storage controller 108 of the content addressable storage system 105is implemented in a distributed manner so as to comprise a plurality ofdistributed storage controller components implemented on respective onesof the storage nodes 115 of the content addressable storage system 105.The storage controller 108 is therefore an example of what is moregenerally referred to herein as a “distributed storage controller.” Insubsequent description herein, the storage controller 108 may be moreparticularly referred to as a distributed storage controller.

Each of the storage nodes 115 in this embodiment further comprises a setof processing modules configured to communicate over one or morenetworks with corresponding sets of processing modules on other ones ofthe storage nodes 115. The sets of processing modules of the storagenodes 115 collectively comprise at least a portion of the distributedstorage controller 108 of the content addressable storage system 105.

The distributed storage controller 108 in the present embodiment isconfigured to implement functionality for decrement protection ofreference counts for inflight small write requests in the contentaddressable storage system 105.

As noted above, the storage devices 106 are configured to store userdata pages 200 and metadata pages 300 in respective user data page andmetadata page areas. Each of the user data pages 200 comprises a logicaladdress and a content-based signature derived from content of that datapage, and each of the metadata pages 300 characterizes a plurality ofthe user data pages 200 and associates the content-based signatures ofthose user data pages with respective physical blocks in the storagedevices 106.

The modules of the distributed storage controller 108 in the presentembodiment more particularly comprise different sets of processingmodules implemented on each of the storage nodes 115. The set ofprocessing modules of each of the storage nodes 115 comprises at least acontrol module 108C, a data module 108D and a routing module 108R. Thedistributed storage controller 108 further comprises one or moremanagement (“MGMT”) modules 108M. For example, only a single one of thestorage nodes 115 may include a management module 108M. It is alsopossible that management modules 108M may be implemented on each of atleast a subset of the storage nodes 115.

Communication links may be established between the various processingmodules of the distributed storage controller 108 using well-knowncommunication protocols such as IP, Transmission Control Protocol (TCP),and remote direct memory access (RDMA). For example, respective sets ofIP links used in data transfer and corresponding messaging could beassociated with respective different ones of the routing modules 108R.

Ownership of a user data logical address space within the contentaddressable storage system 105 is illustratively distributed among thecontrol modules 108C.

The cache 109 of storage system 105 in the FIG. 1 embodiment includeswrite cache entries 109-1, 109-2, . . . , 109-N which store incoming IOrequest data for later destaging to storage devices 106. Cache 109 mayillustratively comprise volatile memory such as, e.g., random accessmemory (RAM), dynamic random-access memory (DRAM), static random-accessmemory (SRAM), or any other kind of volatile memory. In someembodiments, cache 109 may additionally or alternatively comprise anynon-volatile memory as described above with respect to storage devices106. In some embodiments, cache 109 may support a variety of operationsor functions of storage system 105 including, for example, write cache,read cache, temporary metadata storage, or other similar operations.While illustrated as a separate component of storage system 105, in someembodiments, cache 109 may be included as a component of storagecontroller 108. In some aspects, the caches 109 of each storage node 115may operate together as a single cache 109 of the content addressablestorage system 105 where the components of a given storage node 115 mayaccess any portion of the cache 109 including those portions included ascomponents of other storage nodes 115.

It is desirable in these and other storage system contexts to implementfunctionality for decrement protection of reference counts for inflightsmall write requests (“decref protection”) across multiple distributedprocessing modules, such as the processing modules 108C, 108D, 108R and108M of the distributed storage controller 108.

The management module 108M of the storage controller 108 may includedecref protection logic 116 that engages corresponding control logicinstances in all of the control modules 108C and routing modules 108R inorder to implement processes for decrement protection of referencecounts for inflight small write requests within the system 100, as willbe described in more detail below in conjunction with FIGS. 5A-5C.

In some embodiments, the content addressable storage system 105comprises an XtremIO™ storage array suitably modified to incorporatetechniques for decrement protection of reference counts for inflightsmall write requests as disclosed herein. In arrangements of this type,the control modules 108C, data modules 108D and routing modules 108R ofthe distributed storage controller 108 illustratively compriserespective C-modules, D-modules and R-modules of the XtremIO™ storagearray. The one or more management modules 108M of the distributedstorage controller 108 in such arrangements illustratively comprisedecref protection logic 116, although other types and arrangements ofsystem-wide management modules can be used in other embodiments.Accordingly, functionality for decrement protection of reference countsfor inflight small write requests in some embodiments is implementedunder the control of decref protection logic 116 of the distributedstorage controller 108, utilizing the C-modules, D-modules and R-modulesof the XtremIO™ storage array.

In the above-described XtremIO™ storage array example, each user datapage typically has a size of 8 KB and its content-based signature is a20-byte signature generated using an SHA1 hash function. Also, each pagehas a LUN identifier and an offset, and so is characterized by <lun_id,offset, signature>.

The content-based signature in the present example comprises acontent-based digest of the corresponding data page. Such acontent-based digest is more particularly referred to as a “hash digest”of the corresponding data page, as the content-based signature isillustratively generated by applying a hash function such as SHA1 to thecontent of that data page. The full hash digest of a given data page isgiven by the above-noted 20-byte signature. The hash digest may berepresented by a corresponding “hash handle,” which in some cases maycomprise a particular portion of the hash digest. The hash handleillustratively maps on a one-to-one basis to the corresponding full hashdigest within a designated cluster boundary or other specified storageresource boundary of a given storage system. In arrangements of thistype, the hash handle provides a lightweight mechanism for uniquelyidentifying the corresponding full hash digest and its associated datapage within the specified storage resource boundary. The hash digest andhash handle are both considered examples of “content-based signatures”as that term is broadly used herein.

Examples of techniques for generating and processing hash handles forrespective hash digests of respective data pages are disclosed in U.S.Pat. No. 9,208,162, entitled “Generating a Short Hash Handle,” and U.S.Pat. No. 9,286,003, entitled “Method and Apparatus for Creating a ShortHash Handle Highly Correlated with a Globally-Unique Hash Signature,”both of which are incorporated by reference herein.

As mentioned previously, storage controller components in an XtremIO™storage array illustratively include C-module, D-module and R-modulecomponents. For example, separate instances of such components can beassociated with each of a plurality of storage nodes in a clusteredstorage system implementation.

The distributed storage controller in this example is configured togroup consecutive pages into page groups, to arrange the page groupsinto slices, and to assign the slices to different ones of theC-modules.

The D-module allows a user to locate a given user data page based on itssignature. Each metadata page also has a size of 8 KB and includesmultiple instances of the <lun_id, offset, signature> for respectiveones of a plurality of the user data pages. Such metadata pages areillustratively generated by the C-module but are accessed using theD-module based on a metadata page signature.

The metadata page signature in this embodiment is a 20-byte signaturebut is not based on the content of the metadata page. Instead, themetadata page signature is generated based on an 8-byte metadata pageidentifier that is a function of the LUN identifier and offsetinformation of that metadata page.

If a user wants to read a user data page having a particular LUNidentifier and offset, the corresponding metadata page identifier isfirst determined, then the metadata page signature is computed for theidentified metadata page, and then the metadata page is read using thecomputed signature. In this embodiment, the metadata page signature ismore particularly computed using a signature generation algorithm thatgenerates the signature to include a hash of the 8-byte metadata pageidentifier, one or more ASCII codes for particular predeterminedcharacters, as well as possible additional fields. The last bit of themetadata page signature may always be set to a particular logic value soas to distinguish it from the user data page signature in which the lastbit may always be set to the opposite logic value.

The metadata page signature is used to retrieve the metadata page viathe D-module. This metadata page will include the <lun_id, offset,signature> for the user data page if the user page exists. The signatureof the user data page is then used to retrieve that user data page, alsovia the D-module.

Additional examples of content addressable storage functionalityimplemented in some embodiments by control modules 108C, data modules108D, routing modules 108R and management module(s) 108M of distributedstorage controller 108 can be found in U.S. Pat. No. 9,104,326, entitled“Scalable Block Data Storage Using Content Addressing,” which isincorporated by reference herein. Alternative arrangements of these andother storage node processing modules of a distributed storagecontroller in a content addressable storage system can be used in otherembodiments.

Each of the storage nodes 115 of the storage system 105 comprises a setof processing modules configured to communicate over one or morenetworks with corresponding sets of processing modules on other ones ofthe storage nodes. A given such set of processing modules implemented ona particular storage node illustratively includes at least one controlmodule 108C, at least one data module 108D and at least one routingmodule 108R, and possibly a management module 108M. These sets ofprocessing modules of the storage nodes collectively comprise at least aportion of the distributed storage controller 108.

The term “write request” as used herein is intended to be broadlyconstrued, so as to encompass one or more IO operations directing thatat least one data item of a storage system be written to in a particularmanner. A given write request is illustratively received in a storagesystem from a host device. For example, in some embodiments, a writerequest is received in a distributed storage controller of the storagesystem, and directed from one processing module to another processingmodule of the distributed storage controller. More particularly, in theembodiment to be described below in conjunction with FIGS. 5A-5C, areceived write request is directed from a routing module of thedistributed storage controller to a particular control module of thedistributed storage controller. The write request is stored in the writecache portion of cache 109, acknowledged, and subsequently destaged at alater time to a persistent data storage location on one or more ofstorage devices 106. Other arrangements for receiving and processingwrite requests from one or more host devices can be used.

Communications between control modules 108C and routing modules 108R ofthe distributed storage controller 108 may be performed in a variety ofways. An example embodiment is implemented in the XtremIO™ context, andthe C-modules, D-modules and R-modules of the storage nodes 115 in thiscontext are assumed to be configured to communicate with one anotherover a high-speed internal network such as an InfiniBand network. TheC-modules, D-modules and R-modules coordinate with one another toaccomplish various 10 processing tasks.

The logical block addresses or LBAs of a logical layer of the storagesystem 105 correspond to respective physical blocks of a physical layerof the storage system 105. The user data pages of the logical layer areorganized by LBA and have reference via respective content-basedsignatures to particular physical blocks of the physical layer.

Each of the physical blocks has an associated reference count 114 thatis maintained within the storage system, for example, in storage devices106. Reference counts 114 may alternatively be stored or maintained instorage controller 108 or other portions of content addressable storagesystem 105. The reference count 114 for a given physical block indicatesthe number of logical blocks that point to that same physical block.

In releasing logical address space in the storage system, adereferencing operation is generally executed for each of the LBAs beingreleased. More particularly, the reference count 114 of thecorresponding physical block is decremented. A reference count 114 ofzero or another predetermined value, indicates that there are no longerany logical blocks that reference the corresponding physical block, andso that physical block can be released.

The manner in which functionality for decrement protection of referencecounts for inflight small write requests is provided in the FIG. 1embodiment will now be described. The process is assumed to be carriedout by the processing modules 108C, 108D, 108R and 108M. It is furtherassumed that the control modules 108C temporarily store data pages inthe cache 109 of the content addressable storage system 105 and laterdestage the temporarily stored data pages via the data modules 108D inaccordance with write requests received from host devices via therouting modules 108R. The host devices illustratively compriserespective ones of the compute nodes 102 of the computer system 101.

The write requests from the host devices identify particular data pagesto be written in the storage system 105 by their corresponding logicaladdresses each comprising a LUN ID and an offset.

As noted above, a given one of the content-based signaturesillustratively comprises a hash digest of the corresponding data page,with the hash digest being generated by applying a hash function to thecontent of that data page. The hash digest may be uniquely representedwithin a given storage resource boundary by a corresponding hash handle.

The storage system 105 utilizes a two-level mapping process to maplogical block addresses to physical block addresses. The first level ofmapping uses an address-to-hash (“A2H”) table and the second level ofmapping uses a hash-to-physical (“H2P”) table, sometimes known as a hashmetadata (“HMD”) table, with the A2H and H2P tables corresponding torespective logical and physical layers of the content-based signaturemapping within the storage system 105.

The first level of mapping using the A2H table associates logicaladdresses of respective data pages with respective content-basedsignatures of those data pages. This is also referred to as logicallayer mapping.

The second level of mapping using the H2P table associates respectiveones of the content-based signatures with respective physical storagelocations in one or more of the storage devices 106. This is alsoreferred to as physical layer mapping.

For a given write request, both of the corresponding A2H and H2P tablesare updated in conjunction with the processing of that write request.For example, the A2H table may be updated when the page data for thewrite request is stored in cache 109 and the H2P table may be updatedwhen the page data is hardened to storage devices 106 during a destagingprocess.

The A2H and H2P tables described above are examples of what are moregenerally referred to herein as “mapping tables” of respective first andsecond distinct types. Other types and arrangements of mapping tables orother content-based signature mapping information may be used in otherembodiments.

The reference counts 114 mentioned above are illustratively maintainedfor respective physical blocks in the storage devices 106 and each suchreference count 114 indicates for its corresponding physical block thenumber of logical blocks that point to that same physical block. Whenall logical block references to a given physical block are removed, thereference count 114 for that physical block becomes zero or anotherpredetermined value, and its capacity can be released. A given“dereferencing operation” as that term is broadly used herein isintended to encompass decrementing of a reference count 114 associatedwith a physical block.

As mentioned previously, in conjunction with release of logical addressspace in the storage system 105, the storage controller 108 makes thereleased logical address space available to users, executesdereferencing operations for respective ones of the physical blockscorresponding to the released logical address space, and releases anyphysical capacity for which the corresponding reference counts 114 reachzero or another predetermined value.

Techniques for efficient release of logical and physical capacity in astorage system such as storage system 105 are disclosed in U.S. patentapplication Ser. No. 15/884,577, filed Jan. 31, 2018 and entitled“Storage System with Decoupling and Reordering of Logical and PhysicalCapacity Removal,” which is incorporated by reference herein. Suchtechniques may be utilized in illustrative embodiments disclosed herein,but are not required in any particular illustrative embodiment.

The logical address space illustratively comprises one or more ranges oflogical block addresses or LBAs each comprising a LUN ID and an offset.For example, each LBA can identify a particular one of the user datapages 200. The LBAs each correspond to one or more physical blocks inthe storage devices 106. Other types of LBAs and logical address spacescan be used in other embodiments. The term “logical address” as usedherein is therefore intended to be broadly construed.

A given such logical address space may be released responsive todeletion of a corresponding storage volume, snapshot or any otherarrangement of data stored in the storage system 105. Other conditionswithin the storage system 105 can also result in release of logicaladdress space including, for example, snapshot merges, write shadows, orother conditions.

The storage controller 108 illustratively makes the released logicaladdress space available to users in order of released logical address.More particularly, the storage controller 108 can make the releasedlogical address space available to users in order of released logicaladdress by making each of its corresponding released logical addressesimmediately available responsive to that logical address being released.For example, release of one or more LBAs or a range of LBAs by one ormore users can result in those LBAs being made available to one or moreother users in the same order in which the LBAs are released.

The corresponding physical blocks may be released in a different order,through accumulation and reordered execution of dereferencing operationsas described in the above-cited U.S. patent application Ser. No.15/884,577. For example, the storage controller 108 in some embodimentsaccumulates multiple dereferencing operations for each of at least asubset of the metadata pages 300, and executes the accumulateddereferencing operations for a given one of the metadata pages 300responsive to the accumulated dereferencing operations for the givenmetadata page reaching a threshold number of dereferencing operations.

In executing the accumulated dereferencing operations for the physicalblocks, execution of each of the dereferencing operations moreparticularly involves decrementing a reference count 114 of acorresponding one of the physical blocks, and releasing the physicalblock responsive to the reference count 114 reaching a designatednumber, such as zero. Moreover, in executing the accumulateddereferencing operations for the physical blocks, at least a subset ofthe accumulated dereferencing operations are first reordered into anorder that more closely matches a physical layout of the correspondingphysical blocks on the storage devices 106. The reordered dereferencingoperations are then executed in that order.

As a result, the physical blocks may be released in the storage system105 in a different order than that in which their corresponding logicalblocks are released. This provides a number of significant advantages asoutlined in the above-cited U.S. patent application Ser. No. 15/884,577.

Other embodiments can be configured to release physical capacity inother ways. For example, physical capacity in some embodiments can bereleased in the same order in which logical capacity is released.

As indicated above, the storage controller 108, illustrativelycomprising the modules 108C, 108R and 108M as illustrated in FIG. 1 aswell as additional modules such as data modules 108D, is configured toimplement functionality for decrement protection of reference counts forinflight small write requests in the content addressable storage system105.

Execution of a small write IO request received in the storage system 105from a host device illustratively involves the following operations:

1. A synchronous part where the new segment of data is persisted in thewrite cache portion of cache 109 and the IO request is acknowledged.

2. An asynchronous part that destages the new data segment by abackground destager. The construction and hardening of the new data pageis done in this stage by combining the target data page, e.g., the datapage located on storage devices 106 at the mapped location correspondingto the content-based signature, and the new data segment stored in writecache during the synchronous part. The content-based signature of thetarget data page may be determined, for example, based on the receivedsmall write IO request which may specify an address, e.g., LUN+ offset.Using the A2H table, the content-based signature of the target data pagemay be determined.

Since the target data page is combined with the new data segment duringdestaging of the write cache for the small write IO request, it isimportant that the target data page is not removed until a new data pagegenerated based on the combined target data page and new data segmenthas been hardened as a new data page in the storage devices 106. Forexample, the reference count of the target data page should not bedecremented to zero or another predetermined value while a new datasegment targeting the data page is currently pending destaging, e.g., aninflight small write IO request.

As mentioned above, every data page stored in storage devices 106 has areference count 114 that counts the number of references to the page inthe A2H mapping. A page is removed when its reference count 114 isdecremented to zero or another predetermined value. The storagecontroller 108 is responsible for the update of the reference counts 114by sending increment (“Incref”) and decrement (“Decref”) commands to thestorage devices 106. Page reference counts 114 are generally decrementedwhen overwriting an address and when volumes are deleted.

However, there are also logical volume management (LVM) flows that maynot be aware of write cache dependencies and can issue a Decref requestfor a data page even when there are inflight small write IO requesttransactions referencing the content-based signature of the data page,e.g., the content-based signature of the data page targeted by the smallwrite IO request. For example, the LVM component may detect that thecontent-based signature was fully shadowed by all the snapshots thatwere originated from an origin snapshot in a snapshot tree, andconsequently initiate a decrement request to the reference count 114corresponding to this content-based signature. If such decrement commandis executed and reduces the reference count 114 of the target data pageto zero or another predetermined value, the target data page may bedeleted. However, if there are inflight small write IO requests for ashadow write of this content-based signature, a new data page for thissmall write IO request could not be constructed, since the target datapage required for its construction has been deleted. Hence such types offlows may result in data loss.

One solution that prevents the reference count of the target data pagefrom decrementing to zero or another predetermined value while there isan inflight small write IO request targeting the data page is toincrement the reference count of the target data page for any inflightsmall write IO requests that target the data page. However, thissolution may waste a significant amount of processing resources sincesuch an increment operation on the reference count of the target datapage would be performed for every small write IO request, regardless ofwhether the reference count of the target data page will be decrementedby the storage controller 108 while the destaging of the small write IOrequest is pending. In addition, performing an increment operation onthe reference count of the target data page for each inflight smallwrite IO request may also increase IO latency as an additional operationmust be performed during the synchronous part of each IO request processand may be performed on a reference count located at a different nodethus wasting network resources.

In an illustrative embodiment, decref protection logic 116 is disclosedthat addresses these issues by preventing the target data page frombeing deleted before all related small write IO request transactionstargeting that data page are completed, e.g., by persisting a new datapage to the storage devices 106. The decref protection logic 116postpones a Decref request issued by the storage controller 108 for adata page associated with a content-based signature if the data page isreferenced by any inflight small write IO request transactions, untilall corresponding inflight small write IO request transactions arecompleted.

In some illustrative embodiments, only the first Decref transaction forthe data page associated with the content-based signature may bepostponed. In this embodiment, any subsequent Decref transactions may beexecuted normally. For example, since postponing even a single Decreftransaction will prevent the reference count from being decremented tozero or another predetermined value and the target data page from beingdeleted, only one Decref transaction need be postponed to ensure thatthe target data page does not get deleted.

In some illustrative embodiments, all Decref transactions for the datapage associated with the content-based signature may be postponed. Forexample, in this embodiment, no Decref transactions for a target datapage may be allowed to proceed when an inflight write IO request targetsthat data page.

With reference now to FIGS. 1 and 4, in some illustrative embodiments, agiven instance of storage controller 108 comprises decref protectionlogic 116, an associated decref hash table 400, and an associated decrefjournal 118. Decref protection logic 116 implements a process fordecrement protection of reference counts for data pages targeted byinflight small write IO requests that are smaller in size than the pagegranularity of the system. For example, the decref protection logic 116may postpone a Decref transaction that would otherwise decrement thereference count 114 for a data page targeted by a small write IO requestto zero or another predetermined value.

Decref hash table 400 stores an inflight write count 404 and a decrefpostponed flag 406 corresponding to a content-based signature 402, e.g.,hash digest or hash handle, associated with a data page targeted by aninflight small write IO request. For example, the content-basedsignature 402 may be used as an index into decref hash table 400 toaccess the inflight write count 404 and decref postponed flag 406corresponding to the target data page. In an illustrative embodiment,decref hash table 400 may be stored in a volatile memory of controller108, in cache 109, or in other storage of system 105. While decref hashtable 400 is described as a hash table in the illustrative embodiment,any other data structure may be used to store the content-basedsignature 402, inflight write count 404, and decref postponed flag 406.

Inflight write count 404 is a counter that reflects the number ofinflight small write IO request transactions that are overwriting thetarget data page.

Decref postponed flag 406 is a flag indicating whether or not a Decreftransaction was postponed.

Decref journal 118 is a data structure that is stored persistently, forexample, NVRAM of storage system 105, in storage devices 106, or anyother persistent storage associated with storage system 105, and isconfigured to store a content-based signature for a postponed Decreftransaction.

An example process that occurs when a small write IO request is receivedmay be implemented as follows:

1. On receipt of a small write IO request, the content-based signature402, e.g., hash digest, hash handle, or other content-based signature,of the data page targeted by the small write IO request may be used asan index into the decref hash table 400:

-   -   a. If the content-based signature 402 for the target data page        already exists in the decref hash table 400, increment the        inflight write count 404 corresponding to that content-based        signature 402.    -   b. If the content-based signature 402 doesn't exist in the        decref hash table 400, add an entry for the content-based        signature 402 in the decref hash table 400, set the        corresponding inflight write count 404 to 1, and clear the        decref postponed flag 406.

2. When a Decref request is issued by storage controller 108, the Decrefrequest is either executed or postponed according to the followinglogic:

-   -   I. If the content-based signature 402 is found in the decref        hash table 400, e.g., there are inflight small write IO requests        targeting the data page corresponding to that content-based        signature 402:        -   a. If the decref postponed flag 406 is cleared:            -   i. Add the Decref request to the decref journal 118 for                later execution (e.g., adding the content-based                signature associated with the decref request as an entry                in the decref journal 118).            -   ii. Set the decref postponed flag 406 corresponding to                the content-based signature 402 in the decref hash table                400.        -   b. Else (i.e. decref postponed flag 406 is already set):            -   i. Execute the Decref request by decrementing the                reference count 114 for the data page corresponding to                the content-based signature 402.

In some embodiments, when the decref postponed flag 406 is already set,additional Decref requests may also be written to the decref journal118, e.g., accumulated for later execution in decrementing the referencecount 114 of the target data page corresponding to the content-basedsignature 402.

3. On completion of a small write IO Request:

-   -   a. Decrement the inflight write count 404 in the decref hash        table 400 corresponding to the content-based signature 402 of        the data page targeted by the small write IO request.    -   b. If inflight write count 404 is decremented to zero or another        predetermined value, and decref postponed flag 406 is set (i.e.        a Decref request was postponed):        -   i. Remove the Decref request from the decref journal 118.        -   ii. Remove the hash table entry of the decref hash table 400            corresponding to the content-based signature 402 of the data            page.        -   iii. Execute the Decref request for the target data page            corresponding to the content-based signature 402, e.g.,            decrementing the reference count 114 for the target data            page in storage devices 106.

4. On recovery (e.g., after a system failure due to power outage orother event): restore the decref hash table 400 by:

-   -   a. Resetting the decref hash table 400, e.g., by clearing out        any data stored in the hash table or resetting the decref hash        table 400 to its original initialization state.    -   b. Analyzing any recovered write cache transactions, inserting        the corresponding content-based signatures 402 to be protected        into the decref hash table 400, e.g., the content-based        signatures targeted by any recovered small write IO requests,        and incrementing the corresponding inflight write count 404 for        each small write IO request targeting a corresponding        content-based signature 402.    -   c. Analyze any recovered Decref request entries from decref        journal 118 and set the corresponding Decref postpone flag 406        in the decref hash table 400.

The decref protection logic 116 described above guarantees that a datapage is not removed (i.e. decremented to zero or another predeterminedvalue) until all inflight small write IO request transactionsreferencing it are successfully completed, and thus guarantees theconsistency of the second stage of the IO flow. In addition, since theprotection occurs in response to a Decref request instead of for each IOrequest, waste of processing resources may be reduced and IO latency maybe preserved.

The above-described decrement protection of reference counts forinflight small write requests functionality of the storage controller108 is carried out under the control of the decref protection logic 116of the storage controller 108, operating in conjunction withcorresponding control 108C and routing 108R modules, to access the datamodules 108D. The modules 108C, 108D, 108R and 108M of the distributedstorage controller 108 therefore collectively implement an illustrativeprocess for decrement protection of reference counts for inflight smallwrite requests of content addressable storage system 105.

It should also be understood that the particular arrangement of storagecontroller processing modules 108C, 108D, 108R and 108M as shown in theFIG. 1 embodiment is presented by way of example only. Numerousalternative arrangements of processing modules of a distributed storagecontroller may be used to implement functionality for decrementprotection of reference counts for inflight small write requests in aclustered storage system in other embodiments.

Although illustratively shown as being implemented within the contentaddressable storage system 105, the storage controller 108 in otherembodiments can be implemented at least in part within the computersystem 101, in another system component, or as a stand-alone componentcoupled to the network 104.

The computer system 101 and content addressable storage system 105 inthe FIG. 1 embodiment are assumed to be implemented using at least oneprocessing platform each comprising one or more processing devices eachhaving a processor coupled to a memory. Such processing devices canillustratively include particular arrangements of compute, storage andnetwork resources. For example, processing devices in some embodimentsare implemented at least in part utilizing virtual resources such as VMsor Linux containers (LXCs), or combinations of both as in an arrangementin which Docker containers or other types of LXCs are configured to runon VMs.

As a more particular example, the storage controller 108 can beimplemented in the form of one or more LXCs running on one or more VMs.Other arrangements of one or more processing devices of a processingplatform can be used to implement the storage controller 108. Otherportions of the system 100 can similarly be implemented using one ormore processing devices of at least one processing platform.

The computer system 101 and the content addressable storage system 105may be implemented on respective distinct processing platforms, althoughnumerous other arrangements are possible. For example, in someembodiments, at least portions of the computer system 101 and thecontent addressable storage system 105 are implemented on the sameprocessing platform. The content addressable storage system 105 cantherefore be implemented at least in part within at least one processingplatform that implements at least a subset of the compute nodes 102.

The term “processing platform” as used herein is intended to be broadlyconstrued so as to encompass, by way of illustration and withoutlimitation, multiple sets of processing devices and associated storagesystems that are configured to communicate over one or more networks.For example, distributed implementations of the system 100 are possible,in which certain components of the system reside in one data center in afirst geographic location while other components of the cluster residein one or more other data centers in one or more other geographiclocations that are potentially remote from the first geographiclocation. Thus, it is possible in some implementations of the system 100for different ones of the compute nodes 102 to reside in different datacenters than the content addressable storage system 105. Numerous otherdistributed implementations of one or both of the computer system 101and the content addressable storage system 105 are possible.Accordingly, the content addressable storage system 105 can also beimplemented in a distributed manner across multiple data centers.

It is to be appreciated that these and other features of illustrativeembodiments are presented by way of example only, and should not beconstrued as limiting in any way.

Accordingly, different numbers, types and arrangements of systemcomponents such as computer system 101, compute nodes 102, network 104,content addressable storage system 105, storage devices 106, storagecontroller 108 and storage nodes 115 and 120 can be used in otherembodiments.

It should be understood that the particular sets of modules and othercomponents implemented in the system 100 as illustrated in FIG. 1 arepresented by way of example only. In other embodiments, only subsets ofthese components, or additional or alternative sets of components, maybe used, and such components may exhibit alternative functionality andconfigurations. For example, as indicated previously, in someillustrative embodiments a given content addressable storage system orother type of storage system with functionality for decrement protectionof reference counts for inflight small write requests can be offered tocloud infrastructure customers or other users as a PaaS offering.

Additional details of illustrative embodiments will be described belowwith reference to the flow diagrams of FIGS. 5A-5C. FIGS. 5A-5C moreparticularly show example processes for decrement protection ofreference counts for inflight small write requests implemented instorage system such as content addressable storage system 105 of theFIG. 1 embodiment. The content addressable storage system 105 maycomprise a scale-out all-flash storage array such as an XtremIO™ storagearray. A given such storage array can be configured to provide storageredundancy using well-known RAID techniques such as RAID 5 or RAID 6,although other storage redundancy configurations can be used.

The term “storage system” as used herein is therefore intended to bebroadly construed, and should not be viewed as being limited to contentaddressable storage systems or flash-based storage systems.

The storage devices of such a storage system illustratively implement aplurality of LUNs configured to store files, blocks, objects or otherarrangements of data.

A given storage system can be implemented using at least one processingplatform each comprising one or more processing devices each having aprocessor coupled to a memory. Such processing devices canillustratively include particular arrangements of compute, storage andnetwork resources. For example, processing devices in some embodimentsare implemented at least in part utilizing virtual resources such as VMsor LXCs, or combinations of both as in an arrangement in which Dockercontainers or other types of LXCs are configured to run on VMs.

As a more particular example, components of a distributed storagecontroller can each be implemented in the form of one or more LXCsrunning on one or more VMs. Other arrangements of one or more processingdevices of a processing platform can be used to implement a distributedstorage controller and/or its components. Other portions of theinformation processing system 100 can similarly be implemented using oneor more processing devices of at least one processing platform.

The term “processing platform” as used herein is intended to be broadlyconstrued so as to encompass, by way of illustration and withoutlimitation, multiple sets of processing devices and associated storagesystems that are configured to communicate over one or more networks.

The operation of the information processing system 100 will now befurther described with reference to the flow diagrams of theillustrative embodiment of FIGS. 5A-5C. The process as shown in FIG. 5Aincludes steps 502 through 508 and illustrates a synchronous portion ofthe small write request, e.g., the temporary storage of the data segmentassociated with the small write request in cache 109. The process asshown in FIG. 5B includes steps 510 through 518 and illustrates thefunctionality that occurs when a decref request is received. The processas shown in FIG. 5C includes steps 520 through 532 and illustrates anasynchronous portion of the small write request, e.g., the destaging ofthe data segment associated with the small write request from cache 109into storage devices 106. The processes shown in FIGS. 5A-5C aresuitable for use in the system 100 but is more generally applicable toother types of information processing systems each comprising one ormore storage systems. The steps are illustratively performed bycooperative interaction of control logic instances of processing modulesof a distributed storage controller. A given such storage controller cantherefore comprise a distributed storage controller implemented in themanner illustrated in FIGS. 1-4.

With reference now to FIG. 5A, the synchronous portion of the smallwrite request will now be described.

In step 502, small write IO requests are received by storage controller108, for example, from computer system 101 or other host devices. Thesmall write IO requests may include write requests for data segmentsthat are smaller than the page granularity of the storage devices 106.In some embodiments, the storage controller 108 may generate one or moreIO threads to service the small write IO requests.

In step 504, the content-based signatures of data pages targeted by thereceived small write IO requests may be determined, for example, asdescribed above.

In step 506, the IO threads may store the data segments included in thesmall write IO requests in cache 109.

In step 508, the inflight write count 404 stored in decref hash table400 may be incremented for the content-based signatures corresponding toany data pages targeted by the small write IO requests.

With reference now to FIG. 5B, the decref request functionality will nowbe described.

In step 510, storage controller 108 may determine whether a decrementrequest has been issued. In some embodiments, the decrement request maybe issued by the controller 108 in response to another operation. Insome embodiments, the decrement request may be issued by anothercontroller associated with storage controller 108 and received bystorage controller 108, e.g., as part of a distributed system. If nodecrement request has been issued, the process ends.

In step 512, in response to a decrement request being issued, thestorage controller determines whether the decref postponed flag 406 hasbeen set for the corresponding content-based signature 402 in decrefhash table 400.

In step 514, if the decref postponed flag 406 for the correspondingcontent-based signature has not been set in decref hash table 400, thedecref postponed flag 406 is set and the decrement request is postponedin step 516 and the process ends.

In step 518, if the decref postponed flag 406 was determined to alreadybe set in step 512, the decrement request is executed, e.g., thereference count is decremented, and the process ends.

With reference now to FIG. 5C, the asynchronous portion of the smallwrite request will now be described.

In step 520, storage controller 108 completes an inflight write IOrequest, e.g., by performing destaging on the data segment associatedwith the inflight small write IO request that is stored in the cache109. For example, the data segment associated with the small write IOrequest is combined with the target data page and the combined data pagemay be persisted in storage devices 106.

In step 522, in response to completion of a small write IO request, thestorage controller 108 decrements the inflight write count 404 stored indecref hash table 400 at the content-based signature 402 correspondingto the target data page associated with the completed destaged smallwrite IO request.

In step 524, the storage controller 108 determines whether the inflightwrite count 404 has been decremented to zero or to another predeterminedvalue. If the inflight write count 404 has not been decremented to zeroor to another predetermined value, the process ends.

In step 526, if the inflight write count 404 has been decremented tozero or to another predetermined value, storage controller 108determines whether the decref postponed flag 406 for the correspondingcontent-based signature is set.

In step 528, the storage controller 108 removes the entry correspondingto the decref request from the decref journal 118, e.g., thecontent-based signature of the target data page may be removed from thedecref journal 118.

In step 530, if the decref postponed flag 406 for the correspondingcontent-based signature is set the storage controller 108 executes thedecrement request.

In step 532, the storage controller 108 removes the hash table entryincluded the in decref hash table 400 for corresponding content-basedsignature. The process then ends.

Referring back to step 526, if the decref postponed flag 406 is not set,the process proceeds to step 532 and the storage controller 108 removesthe hash table entry included in decref hash table 400 for correspondingcontent-based signature.

It is also to be appreciated that the processes of FIGS. 5A-5C and otherfeatures and functionality for decrement protection of reference countsfor inflight small write requests as described above can be adapted foruse with other types of information systems, including by way of examplean information processing system in which the host devices and thestorage system are both implemented on the same processing platform.

The particular processing operations and other system functionalitydescribed in conjunction with the flow diagrams of FIGS. 5A-5C arepresented by way of illustrative example only and should not beconstrued as limiting the scope of the disclosure in any way.Alternative embodiments can use other types of processing operations forimplementing decrement protection of reference counts for inflight smallwrite requests. For example, the ordering of the process steps may bevaried in other embodiments, or certain steps may be performed at leastin part concurrently with one another rather than serially. Also, one ormore of the process steps may be repeated periodically, or multipleinstances of the process can be performed in parallel with one anotherin order to implement a plurality of different process instances fordecrement protection of reference counts for inflight small writerequests for respective different storage systems or portions thereofwithin a given information processing system.

Functionality such as that described in conjunction with the flowdiagrams of FIGS. 5A-5C can be implemented at least in part in the formof one or more software programs stored in memory and executed by aprocessor of a processing device such as a computer or server. As willbe described below, a memory or other storage device having executableprogram code of one or more software programs embodied therein is anexample of what is more generally referred to herein as a“processor-readable storage medium.”

For example, a storage controller such as storage controller 108 that isconfigured to control performance of one or more steps of the processesof FIGS. 5A-5C can be implemented as part of what is more generallyreferred to herein as a processing platform comprising one or moreprocessing devices each comprising a processor coupled to a memory. Agiven such processing device may correspond to one or more virtualmachines or other types of virtualization infrastructure such as Dockercontainers or other types of LXCs. The storage controller 108, as wellas other system components, may be implemented at least in part usingprocessing devices of such processing platforms. For example, in adistributed implementation of the storage controller 108, respectivedistributed modules of such a storage controller can be implemented inrespective LXCs running on respective ones of the processing devices ofa processing platform.

In some embodiments, the storage system comprises an XtremIO™ storagearray suitably modified to incorporate techniques for decrementprotection of reference counts for inflight small write requests asdisclosed herein.

As described previously, in the context of an XtremIO™ storage array,the control modules 108C, data modules 108D, routing modules 108R andmanagement module(s) 108M of the distributed storage controller 108 insystem 100 illustratively comprise C-modules, D-modules, R-modules andSYM module(s), respectively. These exemplary processing modules of thedistributed storage controller 108 can be configured to implementfunctionality for decrement protection of reference counts for inflightsmall write requests in accordance with the processes of FIGS. 5A-5C.

The techniques for decrement protection of reference counts for inflightsmall write requests implemented in the embodiments described above canbe varied in other embodiments. For example, different types of processoperations can be used in other embodiments.

In addition, the above-described functionality associated with C-module,D-module, R-module and decref protection logic components of an XtremIO™storage array can be incorporated into other processing modules orcomponents of a centralized or distributed storage controller in othertypes of storage systems.

Illustrative embodiments of content addressable storage systems or othertypes of storage systems with functionality for decrement protection ofreference counts for inflight small write requests as disclosed hereincan provide a number of significant advantages relative to conventionalarrangements.

For example, some embodiments can advantageously inhibit the deletion ofdata pages that are required for inflight write IO requests whichprevents data loss. In addition, some embodiments can advantageouslyreduce IO processing waste and latency, for example, by removing theneed to increment the reference count for every data page having anassociated pending write IO request and instead only postponingdecrement requests specifically targeting data pages with inflight writeIO requests.

These and other embodiments include clustered storage systems comprisingstorage controllers that are distributed over multiple storage nodes.Similar advantages can be provided in other types of storage systems.

It is to be appreciated that the particular advantages described aboveand elsewhere herein are associated with particular illustrativeembodiments and need not be present in other embodiments. Also, theparticular types of information processing system features andfunctionality as illustrated in the drawings and described above areexemplary only, and numerous other arrangements may be used in otherembodiments.

As mentioned previously, at least portions of the information processingsystem 100 may be implemented using one or more processing platforms. Agiven such processing platform comprises at least one processing devicecomprising a processor coupled to a memory. The processor and memory insome embodiments comprise respective processor and memory elements of avirtual machine or container provided using one or more underlyingphysical machines. The term “processing device” as used herein isintended to be broadly construed so as to encompass a wide variety ofdifferent arrangements of physical processors, memories and other devicecomponents as well as virtual instances of such components. For example,a “processing device” in some embodiments can comprise or be executedacross one or more virtual processors. Processing devices can thereforebe physical or virtual and can be executed across one or more physicalor virtual processors. It should also be noted that a given virtualdevice can be mapped to a portion of a physical one.

Some illustrative embodiments of a processing platform that may be usedto implement at least a portion of an information processing systemcomprise cloud infrastructure including virtual machines implementedusing a hypervisor that runs on physical infrastructure. The cloudinfrastructure further comprises sets of applications running onrespective ones of the virtual machines under the control of thehypervisor. It is also possible to use multiple hypervisors eachproviding a set of virtual machines using at least one underlyingphysical machine. Different sets of virtual machines provided by one ormore hypervisors may be utilized in configuring multiple instances ofvarious components of the system.

These and other types of cloud infrastructure can be used to providewhat is also referred to herein as a multi-tenant environment. One ormore system components such as storage system 105, or portions thereof,are illustratively implemented for use by tenants of such a multi-tenantenvironment.

As mentioned previously, cloud infrastructure as disclosed herein caninclude cloud-based systems such as AWS, GCP and Microsoft Azure.Virtual machines provided in such systems can be used to implement atleast portions of one or more of a computer system and a contentaddressable storage system in illustrative embodiments. These and othercloud-based systems in illustrative embodiments can include objectstores such as Amazon S3, GCP Cloud Storage, and Microsoft Azure BlobStorage.

In some embodiments, the cloud infrastructure additionally oralternatively comprises a plurality of containers implemented usingcontainer host devices. For example, a given container of cloudinfrastructure illustratively comprises a Docker container or other typeof LXC. The containers may run on virtual machines in a multi-tenantenvironment, although other arrangements are possible. The containersmay be utilized to implement a variety of different types offunctionality within the system 100. For example, containers can be usedto implement respective processing devices providing compute and/orstorage services of a cloud-based system. Again, containers may be usedin combination with other virtualization infrastructure such as virtualmachines implemented using a hypervisor.

Illustrative embodiments of processing platforms will now be describedin greater detail with reference to FIGS. 6 and 7. Although described inthe context of system 100, these platforms may also be used to implementat least portions of other information processing systems in otherembodiments.

FIG. 6 shows an example processing platform comprising cloudinfrastructure 600. The cloud infrastructure 600 comprises a combinationof physical and virtual processing resources that may be utilized toimplement at least a portion of the information processing system 100.The cloud infrastructure 600 comprises multiple virtual machines (VMs)and/or container sets 602-1, 602-2, . . . 602-L implemented usingvirtualization infrastructure 604. The virtualization infrastructure 604runs on physical infrastructure 605, and illustratively comprises one ormore hypervisors and/or operating system level virtualizationinfrastructure. The operating system level virtualization infrastructureillustratively comprises kernel control groups of a Linux operatingsystem or other type of operating system.

The cloud infrastructure 600 further comprises sets of applications610-1, 610-2, . . . 610-L running on respective ones of theVMs/container sets 602-1, 602-2, . . . 602-L under the control of thevirtualization infrastructure 604. The VMs/container sets 602 maycomprise respective VMs, respective sets of one or more containers, orrespective sets of one or more containers running in VMs.

In some implementations of the FIG. 6 embodiment, the VMs/container sets602 comprise respective VMs implemented using virtualizationinfrastructure 604 that comprises at least one hypervisor. Suchimplementations can provide metadata loading control functionality ofthe type described above for one or more processes running on a givenone of the VMs. For example, each of the VMs can implement metadataloading control functionality for one or more processes running on thatparticular VM.

An example of a hypervisor platform that may be used to implement ahypervisor within the virtualization infrastructure 604 is the VMware®vSphere® which may have an associated virtual infrastructure managementsystem such as the VMware® vCenter™. The underlying physical machinesmay comprise one or more distributed processing platforms that includeone or more storage systems.

In other implementations of the FIG. 6 embodiment, the VMs/containersets 602 comprise respective containers implemented using virtualizationinfrastructure 604 that provides operating system level virtualizationfunctionality, such as support for Docker containers running on baremetal hosts, or Docker containers running on VMs. The containers areillustratively implemented using respective kernel control groups of theoperating system. Such implementations can provide metadata load controlfunctionality of the type described above for one or more processesrunning on different ones of the containers. For example, a containerhost device supporting multiple containers of one or more container setscan implement one or more instances of metadata load control logic foruse in loading metadata into cache during a restart process.

As is apparent from the above, one or more of the processing modules orother components of system 100 may each run on a computer, server,storage device or other processing platform element. A given suchelement may be viewed as an example of what is more generally referredto herein as a “processing device.” The cloud infrastructure 600 shownin FIG. 6 may represent at least a portion of one processing platform.Another example of such a processing platform is processing platform 700shown in FIG. 7.

The processing platform 700 in this embodiment comprises a portion ofsystem 100 and includes a plurality of processing devices, denoted702-1, 702-2, 702-3, . . . 702-K, which communicate with one anotherover a network 704.

The network 704 may comprise any type of network, including by way ofexample a global computer network such as the Internet, a WAN, a LAN, asatellite network, a telephone or cable network, a cellular network, awireless network such as a WiFi or WiMAX network, or various portions orcombinations of these and other types of networks.

The processing device 702-1 in the processing platform 700 comprises aprocessor 710 coupled to a memory 712.

The processor 710 may comprise a microprocessor, a microcontroller, anapplication-specific integrated circuit (ASIC), a field-programmablegate array (FPGA) or other type of processing circuitry, as well asportions or combinations of such circuitry elements.

The memory 712 may comprise random access memory (RAM), read-only memory(ROM) or other types of memory, in any combination. The memory 712 andother memories disclosed herein should be viewed as illustrativeexamples of what are more generally referred to as “processor-readablestorage media” storing executable program code of one or more softwareprograms.

Articles of manufacture comprising such processor-readable storage mediaare considered illustrative embodiments. A given such article ofmanufacture may comprise, for example, a storage array, a storage diskor an integrated circuit containing RAM, ROM or other electronic memory,or any of a wide variety of other types of computer program products.The term “article of manufacture” as used herein should be understood toexclude transitory, propagating signals. Numerous other types ofcomputer program products comprising processor-readable storage mediacan be used.

Also included in the processing device 702-1 is network interfacecircuitry 714, which is used to interface the processing device with thenetwork 704 and other system components, and may comprise conventionaltransceivers.

The other processing devices 702 of the processing platform 700 areassumed to be configured in a manner similar to that shown forprocessing device 702-1 in the figure.

Again, the particular processing platform 700 shown in the figure ispresented by way of example only, and system 100 may include additionalor alternative processing platforms, as well as numerous distinctprocessing platforms in any combination, with each such platformcomprising one or more computers, servers, storage devices or otherprocessing devices.

For example, other processing platforms used to implement illustrativeembodiments can comprise different types of virtualizationinfrastructure, in place of or in addition to virtualizationinfrastructure comprising virtual machines. Such virtualizationinfrastructure illustratively includes container-based virtualizationinfrastructure configured to provide Docker containers or other types ofLXCs.

As another example, portions of a given processing platform in someembodiments can comprise converged infrastructure such as VxRail™,VxRack™, VxRack™ FLEX, VxBlock™ or Vblock® converged infrastructure fromVCE, the Virtual Computing Environment Company, now the ConvergedPlatform and Solutions Division of Dell EMC.

It should therefore be understood that, in other embodiments, differentarrangements of additional or alternative elements may be used. At leasta subset of these elements may be collectively implemented on a commonprocessing platform, or each such element may be implemented on aseparate processing platform.

Also, numerous other arrangements of computers, servers, storage devicesor other components are possible in the information processing system100. Such components can communicate with other elements of theinformation processing system 100 over any type of network or othercommunication media.

As indicated previously, components of an information processing systemas disclosed herein can be implemented at least in part in the form ofone or more software programs stored in memory and executed by aprocessor of a processing device. For example, at least portions of thefunctionality of one or more components of the storage controller 108 ofsystem 100 are illustratively implemented in the form of softwarerunning on one or more processing devices.

It should again be emphasized that the above-described embodiments arepresented for purposes of illustration only. Many variations and otheralternative embodiments may be used. For example, the disclosedtechniques are applicable to a wide variety of other types ofinformation processing systems, storage systems, storage nodes, storagedevices, storage controllers, processing modules, decrement protectionprocesses and associated control logic. Also, the particularconfigurations of system and device elements and associated processingoperations illustratively shown in the drawings can be varied in otherembodiments. Moreover, the various assumptions made above in the courseof describing the illustrative embodiments should also be viewed asexemplary rather than as requirements or limitations of the disclosure.Numerous other alternative embodiments within the scope of the appendedclaims will be readily apparent to those skilled in the art.

What is claimed is:
 1. An apparatus comprising: a storage systemcomprising a plurality of storage devices and an associated storagecontroller; the plurality of storage devices being configured to store aplurality of data pages, each of the data pages having a content-basedsignature derived from content of that data page, the content-basedsignatures of the data pages being associated with physical locations inthe plurality of storage devices where the data pages are stored, theplurality of storage devices storing a reference count for each physicallocation, a given reference count indicating a number of the data pagesthat map via their respective content-based signatures to the samephysical location in the plurality of storage devices; wherein thestorage controller is configured to: receive a write input/output (IO)request, the write IO request comprising a data segment smaller than apage granularity of the plurality of storage devices; in response toreceiving the write IO request, determine a content-based signatureassociated with the data segment, the content-based signaturecorresponding to a target data page stored at one of the physicallocations; and in response to a decrement request to decrement areference count of the physical location corresponding to thecontent-based signature of the target data page, postpone the decrementrequest, wherein the storage controller is implemented using at leastone processing device comprising a processor coupled to a memory.
 2. Theapparatus of claim 1, wherein the storage controller is furtherconfigured to: in response to determining the content-based signatureassociated with the data segment, increment an inflight write countcorresponding to the determined content-based signature of the targetdata page in a data structure associated with the storage controller. 3.The apparatus of claim 2, wherein the storage controller is furtherconfigured to: in response to completion of the received write IOrequest, decrement the inflight write count; and in response to theinflight write count being decremented to a predetermined value, executethe postponed decrement request.
 4. The apparatus of claim 3, whereinthe storage controller is further configured to: in response to theinflight write count being decremented to the predetermined value,remove an entry in the data structure corresponding to the content-basedsignature of the target data page from the data structure, the entryincluding the inflight write count corresponding to the determinedcontent-based signature of the target data page.
 5. The apparatus ofclaim 2, wherein incrementing the inflight write count corresponding tothe determined content-based signature of the target data page in thedata structure comprises determining whether the data structure includesan entry corresponding to the content-based signature of the target datapage, wherein in response to determining that the data structureincludes an entry corresponding to the content-based signature of thetarget data page, the entry including the inflight write countcorresponding to the determined content-based signature of the targetdata page, the storage controller is configured to increment theinflight write count in the entry, and wherein in response todetermining that the data structure does not include an entrycorresponding to the content-based signature of the target data page,the storage controller is configured to add an entry corresponding tothe content-based signature of the target data page to the datastructure, set an inflight write count in the added entry to 1, andclear a decrement postponed flag in the added entry.
 6. The apparatus ofclaim 1, wherein the storage controller is further configured to: inresponse to the decrement request, set a decrement postponed flagcorresponding to the content-based signature of the target data page ina data structure associated with the storage controller; in response toa second decrement request to decrement the reference count of thephysical location corresponding to the content-based signature of thetarget data page: determine whether the decrement postponed flagcorresponding to the content-based signature of the target data page isset in the data structure; and in response to determining that thedecrement postponed flag corresponding to the content-based signature ofthe target data page is set in the data structure, decrement thereference count of the physical location corresponding to thecontent-based signature of the target data page.
 7. The apparatus ofclaim 2, wherein postponing the decrement request comprises storing thedecrement request in a decrement request postponed journal incorrespondence with the content-based signature of the target data page.8. The apparatus of claim 7, wherein in response to a recovery of thestorage system after an event, the storage controller is configured to:reset the data structure; determine whether any recovered write IOrequests include a data segment smaller than a page granularity of theplurality of storage devices; for a given write IO request that includesa data segment smaller than a page granularity of the plurality ofstorage devices, increment the inflight write count corresponding to thecontent-based signature of a data page targeted by the given write IOrequest in the data structure; determine whether the decrement requestpostponed journal includes a decrement request corresponding to thecontent-based signature of the data page targeted by the given write IOrequest; and in response to determining that the decrement requestpostponed journal includes a decrement request corresponding to thecontent-based signature of the data page targeted by the given write IOrequest, set a decrement postponed flag corresponding to thecontent-based signature of the data page targeted by the given write IOrequest in the data structure.
 9. The apparatus of claim 7, wherein thestorage controller is further configured to: in response to a decrementof the inflight write count corresponding to the content-based signatureof the target data page to a predetermined value, execute the postponeddecrement request and remove the decrement request corresponding to thecontent-based signature of the target data page from the decrementrequest postponed journal.
 10. A method comprising: receiving a writeinput/output (IO) request, the write IO request comprising a datasegment smaller than a page granularity of a plurality of storagedevices, the plurality of storage devices being configured to store aplurality of data pages, each of the data pages having a content-basedsignature derived from content of that data page, the content-basedsignatures of the data pages being associated with physical locations inthe plurality of storage devices where the data pages are stored, theplurality of storage devices storing a reference count for each physicallocation, a given reference count indicating a number of the data pagesthat map via their respective content-based signatures to the samephysical location in the plurality of storage devices; in response toreceiving the write IO request, determining a content-based signatureassociated with the data segment, the content-based signaturecorresponding to a target data page stored at one of the physicallocations; and in response to a decrement request to decrement areference count of the physical location corresponding to thecontent-based signature of the target data page, postponing thedecrement request, wherein the method is implemented by at least oneprocessing device comprising a processor coupled to a memory.
 11. Themethod of claim 10, wherein the method further comprises: in response todetermining the content-based signature associated with the datasegment, incrementing an inflight write count corresponding to thedetermined content-based signature of the target data page in a datastructure associated with the storage controller.
 12. The method ofclaim 11, wherein the method further comprises: in response tocompletion of the received write IO request, decrementing the inflightwrite count; and in response to the inflight write count beingdecremented to a predetermined value, executing the postponed decrementrequest.
 13. The method of claim 12, wherein the method furthercomprises: in response to the inflight write count being decremented tothe predetermined value, remove an entry in the data structurecorresponding to the content-based signature of the target data pagefrom the data structure, the entry including the inflight write countcorresponding to the determined content-based signature of the targetdata page.
 14. The method of claim 11, wherein incrementing the inflightwrite count corresponding to the determined content-based signature ofthe target data page in the data structure comprises determining whetherthe data structure includes an entry corresponding to the content-basedsignature of the target data page, wherein in response to determiningthat the data structure includes an entry corresponding to thecontent-based signature of the target data page, the entry including theinflight write count corresponding to the determined content-basedsignature of the target data page, the storage controller is configuredto increment the inflight write count in the entry, and wherein inresponse to determining that the data structure does not include anentry corresponding to the content-based signature of the target datapage, the storage controller is configured to add an entry correspondingto the content-based signature of the target data page to the datastructure, set an inflight write count in the added entry to 1, andclear a decrement postponed flag in the added entry.
 15. The method ofclaim 10, wherein the method further comprises: in response to thedecrement request, setting a decrement postponed flag corresponding tothe content-based signature of the target data page in a data structureassociated with the storage controller; in response to a seconddecrement request to decrement the reference count of the physicallocation corresponding to the content-based signature of the target datapage: determining that the decrement postponed flag corresponding to thecontent-based signature of the target data page is set in the datastructure; and in response to determining that the decrement postponedflag corresponding to the content-based signature of the target datapage is set in the data structure, decrementing the reference count ofthe physical location corresponding to the content-based signature ofthe target data page.
 16. The method of claim 11, wherein postponing thedecrement request comprises storing the decrement request in a decrementrequest postponed journal in correspondence with the content-basedsignature of the target data page.
 17. The method of claim 16, whereinin response to a recovery of a storage system comprising the pluralityof storage devices after an event, the method further comprises:resetting the data structure; determining whether any recovered write IOrequests include a data segment smaller than a page granularity of theplurality of storage devices; for a given write IO request that includesa data segment smaller than a page granularity of the plurality ofstorage devices, incrementing the inflight write count corresponding tothe content-based signature of a data page targeted by the given writeIO request in the data structure; determining whether the decrementrequest postponed journal includes a decrement request corresponding tothe content-based signature of the data page targeted by the given writeIO request; and in response to determining that the decrement requestpostponed journal includes a decrement request corresponding to thecontent-based signature of the data page targeted by the given write IOrequest, setting a decrement postponed flag corresponding to thecontent-based signature of the data page targeted by the given write IOrequest in the data structure.
 18. The method of claim 16, wherein themethod further comprises: in response to a decrement of the inflightwrite count corresponding to the content-based signature of the targetdata page to a predetermined value, executing the postponed decrementrequest and removing the decrement request corresponding to thecontent-based signature of the target data page from the decrementrequest postponed journal.
 19. A computer program product comprising anon-transitory processor-readable storage medium having stored thereinprogram code of one or more software programs, wherein the program codewhen executed by at least one processing device causes said at least oneprocessing device to: receive a write input/output (IO) request, thewrite IO request comprising a data segment smaller than a pagegranularity of a plurality of storage devices, the plurality of storagedevices being configured to store a plurality of data pages, each of thedata pages having a content-based signature derived from content of thatdata page, the content-based signatures of the data pages beingassociated with physical locations in the plurality of storage deviceswhere the data pages are stored, the plurality of storage devicesstoring a reference count for each physical location, a given referencecount indicating a number of the data pages that map via theirrespective content-based signatures to the same physical location in theplurality of storage devices; in response to receiving the write IOrequest, determine a content-based signature associated with the datasegment, the content-based signature corresponding to a target data pagestored at one of the physical locations; and in response to a decrementrequest to decrement a reference count of the physical locationcorresponding to the content-based signature of the target data page,postpone the decrement request.
 20. The computer program product ofclaim 19, the program code when executed by at least one processingdevice further causes said at least one processing device to: inresponse to determining the content-based signature associated with thedata segment, increment an inflight write count corresponding to thedetermined content-based signature of the target data page in a datastructure associated with the storage controller; in response tocompletion of the received write IO request, decrement the inflightwrite count; and in response to the inflight write count beingdecremented to a predetermined value, execute the postponed decrementrequest.